This invention relates to die cast machines in which a plunger is employed to force molten metal to be cast through a shot cylinder (cold chamber) and into a mold, and in particular to providing longer lives, reduced costs and lower maintenance for plunger tips for plungers in such machines.
As is well known in the art of die casting, molten metal is forced into a die cavity under pressure exerted by a plunger. The tip of the plunger, being subjected to high pressures, friction and contact with molten metal, is subject to substantial wear. Prior plunger tips have been one-piece affairs, simply removably attached to the end of the plunger shaft.
In hot chamber die cast machines, the plunger tips generally have a number of split rings for providing sealing, much as shown in Thompson, U.S. Pat. No. 3,300,822, except that prior to Thompson the split rings were rectangular in cross section. The Thompson patent, on the other hand, shows a plunger tip having a number of split rings, the cross section of which is not strictly rectangular but having a chamfered surface toward the molten metal, ostensibly to provide self-lubrication of the rings against the interior wall of the shot cylinder. Hot chamber die cast apparatus, such as disclosed in Thompson, however, are not properly applicable to cold chamber machines, because the problems encountered are substantially different. For instance, passage of molten metal beyond the sealing rings is a less severe problem in hot chamber die casting since the metal will remain molten. In cold chamber die casting, however, such leakage would cause "canning", freezing any such split rings and preventing their flexure, thereby increasing friction and wear, rather than decreasing it.
The referenced Thompson patent proposes the solution of lubricating the tip, in hot chamber die casting. It has been found by the inventor of the present invention, though, that in cold chamber die casting lubrication is not as great a problem as leakage of the molten metal past a portion of the rings, causing excessive wear of the rings and of the tip itself, requiring premature replacement of the entire tip. Further, in considering pressures that become very high in the cold chamber casting process, splits in the rings can be another source of leakage of molten metal and premature wear. Such leaked molten metal freezes around the ring, preventing it from flexing, and thus preventing it from providing proper seal.
The patent issued to Morton, U.S. Pat. No. 3,092,880, shows a three piece plunger tip where a tip body is screwed onto the plunger rod, a sealing ring is interlocked into the tip body, and a plunger cap is press fitted into the sealing ring. The disadvantage of this assembly is that the cap is in effect merely press fitted to the plunger rod. Since the cap must at one step of the casting cycle disclosed in that patent exert a vacuum on the molten metal, or more commonly be pulled free from solidified metal adhered to the cap, it could conceivably pull free, resulting in substantial down time in thereafter removing the cap from the apparatus or from the hardened metal. If the press fit is indeed made quite secure to avoid this problem, replacement of the sealing ring while allowing re-use of the cap is very difficult, and it is generally required to replace the cap along with the sealing ring, which is wasteful.
A Japanese patent, 57-068257, shows a plunger tip wherein a cap is threadedly attached to a body, with a number of split rings therebetween. The body is in turn threadedly attached to a plunger rod. A thermal boundary between the parts effects cooling. However, the statements above relating to split rings apply to this device. Further, threading the cap onto the body requires the application of a wrench to the cap, and any resulting damage to the cap, or any flats provided for that purpose which are exposed to the molten metal, will also be a point of potential sticking for the solidifying metal in front of the tip, again causing premature wear and unnecessarily shortening the life of the tip assembly.
This invention relates to improvements over the apparatus described above and to solutions to the problems raised thereby.